Power controller for pneumatic-dispatch systems



J. G. MACLAREN POWER GOITROLLER FOR PNEUHATIC DISPATCH SYSTEMS FiledDec. 5, 1924- QUCT/ON DRUM InvenZQF 05 7,1285 Cijfcza7'e7z, 9- 49 012.1:@1:

Patented June 23, 1925.

UNITED STATES PATENT OFFICE.

JIIMES G. MACLAR-EN, oF MAmARONECK, NEW YORK, ASSIGNOR TO THE LAMSONCOMPANY, OF SYRACUSE, NE? YORK, A CORPORATION OF MASSACHUSETTS.

Application filed December 3, 1924. Serial No. 753,637.

T 0 all whom it may concern.

Be it known that I, JAMES G. lVIACLAREN, a citizen of the United Statesof America, and resident of Mamaroneck, in the county of lVestchesterand State of New York, have invented new and useful Improvements inPower Controllers for Pneumatic-Dispatch Systems, of which the followingis a specification. v

This invention pertains to pneumatic dispatch systems of the so-calledminimum flow vacuum type and relates more particularly to automaticpower controlling apparatus for use in such a system.

Power control apparatus of this general character is disclosed in thepatent to Libby No. 968,576, dated August 30, 1910. The patentedapparatus provides automatic pressure actuated means for initiating acarrier propelling air flow through the transmission tube whenever acarrier is entered therein as well as means for cutting off such flowafter the lapse of a predetermined period of time. While I regard thepatented device as generally correct in principle and highly effectivein practice I believe it preferable to have the cessation of carrierpropelling air flow depend directly upon the discharge of the carrierfrom the delivery end of the tube rather than upon the operation of atiming device, and the principal object of this invention is to devisean improved power control apparatus comprising means actuableautomatically upon insertion of a carrier withinthe dispatch terminal toinitiate flow of the air propelling current and also operatingautomatically when the last carrier emerges from the delivery terminalto cut off the air current without appreciable delay.

In the accompanying drawings I have disclosed one preferredembodiment ofthe invention by way of example i,

Fig. 1 being a fragmentary View illustrating a part of the centralstation desk of a pneumatic system having the improved power controlapparatus applied thereto; Fig. 2 being a verticalsection diametricallythrough the suction tubeof the control apparatus, the controlling valvebeing shown in normal or closed position;

Fig. 8 being a view similar to Fig. 9- but with the controlling valveopen but just on the point of closing; I

Fig. 4- being a section substantially on the line 4 l of Fig. 2;

Fig. 5 being a section on the line 5 5 of Fig 3; and

Fig. Gbeing a diagram illustrative of the character of the pressurecycles occurring in control apparatus of the general'type to which thepresent invention relates.

Referring to the drawings, the numeral 1 indicates a fragmentary portionof the central station desk of a pneumatic dispatch system comprisingtransmission tubes, such for example as the tubes 2, which lead from thecentral station to remote sending stations and then back to the centralstation desk. Each tube 2 is furnished with a delivery terminal 3 and isprovided with a continuation t connecting it to an individual powercontrol device indicated generally by the numeral 5. This power control.device is connected in turn by means of a suction tube 7 to a vacuumdrum or header 8 in which a low pressure is continually maintainedbymeans of a suitable pump, exhauster or equivalent apparatus, notshown.

An open ended dispatch terminal 6 is provided at the central station foreach of the transmission'tubes 2, each dispatch terminal forming theentrance to that run of the tube which leads to the corresponding remotesending station. The above arrangement ingeneral is old and well knownin this art and further details of such general arrangement aretherefore unnecessary, the present invention being confined to-improvements in the power control apparatus per se.

Referring particularly to Figs. 2 and 3, the power control apparatus isshown as comprising a hollow casing having an elongate body. portion 9provided with a passage adapted to connect the transmission,

tube 4 with the suction 7. The upper end of the'casing 9 is preferablyprovided with a flange 10 for the reception of the lower end of the tube4 while the lower end of the casing is likewise provided with afiange 11for the reception of the upper end of the tube 7.

For convenience in description the upper part 12 of the passage in thebody portion 9 of the casing is hereinafter referred to as the inlet ortransmission chamber, while the lower part 13 of this passage isreferred to as the suction or outlet chamber.

The upper part of the casing 9 is turnished with oppositely directedcylindrical hollow arms 14 and 15 respectively having their axes insubstantial alignment and at right angles to the axis of the casing 9.These hollow arms form a guideway for a sliding piston valve comprisingthe cylindrical shell or valve proper 15 and the head 16. This pistonvalve fits snugly within the space 17 in the hollow arm 14 and the head16 thus forms a movable wall for such space.

The piston valve is also provided with a hollow stem 16 coaxial with thehead 16 and projecting out through the open end of the shell 15 into thehollow arm 15. At its outer end this sleeve 16 is furnished with asecondary head 18 having a sliding fit against the inner surface of thearm 15 to assist in guiding the shell 15 in its movements.

A coiled tension spring 20 is disposed within the hollow stem 16 and hasone end attached to the head 16 oi the piston valve by means of a stud21. The opposite end of the spring is connected to a screw 22 engaging athreaded opening in the cap or head 23 which forms a closure for theouter end of the hollow arm 15. This cap or head 23 is also furnishedwith a threaded opening for the reception of a stop screw 2 1 whichprojects into the interior of the arm 15 and is 'engageable by the head18 of the piston valve to limit movement of the latter in one directionunder the action of the spring 20.

This stop screw 24 is so set that when the head 18 contacts therewith asmall space or crevice 25 is provided between the edge of the valveshell 15 and the adjacent wall of the casing 9. This crevice providesfor the normal minimum fiow of air from the inlet to the outlet chamberof the casing which is necessary for the proper operation of controlapparatus of this general type. By the provision of the adjustable stop2 1 this minimum flow of air past the closed valve may be varied asdesired.

' The cap 23 closing the hollow arm 15 is also furnished with an opening26 to admit atmospheric pressure to the space between such cap and thehead 18 of the piston valve. In order to prevent the piston valve fromrotating within its cylindrical guideway, a rod 19 is disposedlongitudinally of the guideway in which the piston valve slides, thelatter having openings for the passage of the rod. This rod is securedin fixed position eccentrically of the guidewa y and this prevents thevalve from rotating.

The space 17 in the hollow arm 14; is connected to the inlet chamber ofthe casing by means of a passage 27 provided in a suitable boss 27 onthe upper side of the arm. The capacity or cross sectional area of thispassage may be varied by means of an adjustable screw 28. For limitingthe degree of opening of the piston valve an adjustable stop screw 29 isprovided in the outer closed end of the arm 14.

The space 17 communicates with the at mosphere by means of passages 3Oand 30 in the boss 27 these passages merging into a single passage 30which leads to a valve orifice 31 communicating with the outeratmosphere by means 01' passages 32- a-nd 33 (see Fig. 3).

The valve orifice 31 is normally closed by means of an auxiliary valve341- secured to a valve stem 35 carried by a piston 36. This pistonslides in a cylindrical casing 37 which may form an integral part of themain casing of the control. apparatus, or may as herein shown, he formedas a separate member and connected to the main casing by means of screws38.

This casing 37 is provided with spaced heads 39 and -10 respectively.The space 11 between the head 39 and the piston 36 is connected by meansof a passage 42 with the outlet chamber 13 of the body portion of thecasing. The chamber 43 at the op posite side of the piston 36communicates with the outer atmosphere by means of orifices 44. and asin the head 40. The orifice 44 is freely open at all times but is verysmall, so that the passage of air therethrough is greatly restricted.The orifice 4-5 is much larger than the orifice 1 1 and is provided witha check valve 46 which permits tree discharge of air from within thecasing 37 but which substantially prevents entrance of air through theorifice 45.

A light coiled spring 47 surrounds the valve stem 5, being interposedbetween the head 39 of the casing and the head 36 of the piston. Thisspring thus tends to move the piston to the right as illustrated in thedrawings and to open the valve 34:, thus to permit free entrance of airat atmospheric pressure into the space 17.

In order to obtain a clear understanding of the mode of operation ofthis device it is desirable to consider the cycles of pressure variationoccurring in the inlet and outlet chambers of minimum flow controlvalves of the generaltype to which this invention pertains. These cyclesare illustrated in a diagrammatic manner in Fig. 6 wherein the ordinatesrepresent suction .or vacuum in ounces or inches of water for examplewhile abscissae indicate time intervals.

In this diagram thesolid line is intended to indicate pressureconditions in the transmission or inlet chamber of the casing, while thedotted line indicates pressure conditions in the suction or outletchamber of the casing. 1

Assuming that the main valve is closed and that a minimum flow of air.is taking place from the inlet chamber to the outlet chamber the suctionin the outlet'chamber will be but slightly below that of the vacuumdrum, as indicated-for example at 51 in the-diagram. On the other hand,the suction .in the inlet chamber will be almost nil due to therestricted character of the passage leading from the suction chamber tothe transmission chamber. This inlet chamber suction may be indicated atin the diagram.

When a carrier is introduced into the transmission tube the freeentrance of air into the latter is thus obstructed so that the bleeding.ofair from the" tube through the minimum flow passage gradually reducesthe pressure in the transmission tube. The point 52 in the diagram isintended to indicatethe time at which the carrier is introduced and fromthis point the suction gradually rises until it reaches some point suchas 54 in the diagram.

The insertion of the carrier in the transmission tube and therestriction of the free entrance of air into the latter may also have aslight efl'ect upon the pressure in the suc tion or outlet end of thecasing for the reason that a lesser quantity of air is admitted to thisend of the casing through the minimum flow passage. However, as this endof the casing isv directly connected to the vacuum drum, the drop inpressure'is'relatively slight. On. the line indicating suction in theoutlet end of the casing the point 53 corresponds to the entrance of thecarrier into the transmission tube and from this point the vacuumgradually increases to the point 55, substantiallyas shown.

In the specific embodiment here shown, when the vacuum or suction in theinlet chamber 12 reaches the point indicated at 54 in the diagram, themain controlling valve-is opened in opposition to the action of its'spring2O by reason of the differential pressures acting upon the heads16 and 18' of the valve, the reduced pressure of the inlet chamber beingcommunicated to the space 17 through the passage 27. I

Immediately upon opening of the main valve the transmission tube and thesuction tube are placed in direct communication so that the suction inthe transmission tube tends to rise rapidly, while the suction in thesuction end of the casing rapidly falls until these suctions becomesubstantially equal, as indicated on the diagram at the point 56. Thevacuum orsuction at this time existing in the casing 9 is somewhat lessthan that of the suction drum, one reason for this being .the fact thatthe carrier is quite free to move in the tube. The car rier now takes upits flight and during its flight the suction or vacuum in the tube maydrop slightly-due to the constantly increasing momentum of the carrier,this drop being indicated in the diagram at 57.

When the carrier emerges from the transmission tube the latter is leftfreely open to the atmosphere so that the vacuum orsuction in the tubeand casing immediately drops rapidly and approaches atmosphericpressure; However, as the vacuum drum is of a capacity greatly exceedingthe capacity of the transmission tube, a certain, degree of vacuum isstillmaintained in the latter,

even when it is freely open to the atmosphere', and the vacuum orsuction thus maintained while the tube is freely open is greater thanthat which is necessary to open the controlling valve when the carrieris first introduced and While the .main valve is closed. The controllingvalve under such circumstances will thus remain open indefinitely unlesssome other means be provided for varying the pressure conditions in thecasing. i

In the device herein disclosed the piston 36 is exposed upon its outerface to atmospheric. pressure, whileits inner face is exposed to thesuction obtaining in the chamber 13. When the main valve is closed thissuction is suflicient to hold the piston 36 in the position shown inFig. 2, compressing the light spring 47 and holding the valve 3.4closed. Also, after the main valve has opened and during flight of thecarrier, although the suction in the chamber 13 decreases to averyconsiderable extent, it is still suflicient to maintainthe valve 36in this initial position. When :the carrier emerges from thetransmission tube the suction both in the inlet chamber 12 and in theoutlet or chamber 13 of the casing drops very rapidly from thepoint'58'in the diagramuntil some point such as 59 is reached at whichthe spring 47 overcomes atmospheric'pressure acting upon the outer sideofthe piston 36 and opens the'valve 34, although at this time-thesuction in the inlet chamber and space 17 is still sufficient to holdthe main valve open.

When the valve 34 opens it permits air at atmospheric pressure freely toenter the space 17 so that the pressure in such space immediately risesand permits the spring 20 to. close the main valve. As soon as the mainvalve begins to close the suction in the chamber 13 begins to rise againthus tendingto .pull the piston 36 back to the position shownin-Fig. 2.In order to prevent this piston from closing the valve 34 too quicklyand thus cutting off atmospheric pressure from the .space 17 before thevalve 16 has completely closed, the admission of air into the chamber 43behind the piston 36 is restricted (by the small capacity of orifice 44)so that the piston 36 moves slowly back to its normal position; Thevalve 16 is thus closed with certainty and the parts are restored tonormal position. As soon as the main. valve iully closes, pressure inthe inlet chamber returns to that resuitingfrom minimum flow through thecasing while the pressure in the suction chamber likewise becomessubstantially that of the suction drum.

iWith the construction thus provided there is no necessity for theprovision :of any timing arrangement, and the closure of the main valveis dependent wholly upon the delivery of the last carrier from thetransmission tube. The structure provided for producing the desiredresult is of simple character but strong and durable and operates withthe utmost certainty/under all conditions of use, whether thetransmission tube be long .or short or whether it be straight orprovided with abrupt turns, or long vertical descents.

As herein disclosed the main and auxiliary valves are actuated bypistons but it is contemplated that any other type of pressure operatedactuator .or pneumatic moved by differences in pressure at its oppositesides may .be substituted "for such pistons. Further, while I haveillustrated the invention as embodied in a single specific construction,it is to be understood in general that in its broader aspect theinvention is :not in any way confined to this particular embodiment, butthat various changes .and modifications in structural arrangement,proportions of parts and constituent elements employed may :be made .asoccasion may warrant, without departing from the spirit of .the appendedclaims. v

I claim:

l. A power control apparatus for a pneumatic dispatch system of :thevacuum, minimum flow :type :having a transmission ztube andan exhauster,.said apparatus having chambers communicating respectively with thetransmission tube and the exhauster, a

.normally closed valve controlling a passage leading from one o'fsaidchambers to :the other, a minimum :fiOW from one chamber to .the otherbeing permitted when the valve is closed, and means sensitivelyresponsive to pressure change OCCIIITIIIg :1n the chamber communicatingwith the exhauster to 1111- -tiate movement of the valve in onedirecinatic idispatch system of :the vacuum, mini- .mum flo-w typehaving a transmission "tube and an exhauster, said apparatus havingspaces, and means immediately and responsive chambers communicating withthe transmission tube and eXh-auster respectively, a normally closedvalve controlling communication between said chambers, :a flow of airfrom one chamber to the other being permitted when the valve is closed,and means tor determining movement of said valve comprising elementssensitively responsive respectively to varying conditions inthe'respective chambers incident to introduction into and discharge of acarrier from the transmission tube.

3. Power control apparatus for 1a pneumatic dispatch system of thevacuum, mimmum flow type "having :a transmission tube and an exhauster,said apparatus having chambers communicating respectively with said tubeand exhauster, a normally closed valve controlling a passage connectingthe chambers, a minimum flow of :air from one chamber .to the otherbeing permitted when the valve is closed, and means'ior moving the valvecomprising pressure operated elements sensitively responsiverespectively- 'to pressure fluctuations in the respective chambers. v

l. A power control apparatus for arpneumatic dispatch system of thevacuum minimum flow type, said apparatus comprising a conduit providedwith a normally closed main valve, a minimum How of airiromone side tothe other of the closed va lve being permitted, the conduit havingcpposite sides respectively of :the valve in spaces upon which thepressure fluctuates ,diiierently during a complete cyclebf opening andclosing of .the valve, means for opening the main valve constructed :andarranged to initiate its opening upon the occurrence of a predeterminedpressure in one of said to the subsequent occurrence of a predeterminedpressure in the other of said spaces to initiate closing of the mainvalve.

5. A power control apparatus of .theminimum flow vacuum type comprisinga conlduit provided with a. normally closed main valve and a chamber ateither .side-cfsaid valve, .a minimum how of air from one chamber to theother being permitted when the main valve is closed, the air pressure inthe respective chambers fluctuating in accordance with difierentsequences during the -.operat1on.of the apparatus, meansfor opening themain valve constructed and arranged toinitiate its opening upon theoccurrence of a predetermined decrease in :pressure in one of saidchambers, and means for initiating closure of tlie'main valve upon thesubsequent occurrence, during the cycle of pressure *fiHClZlOIlS, of apredetermined increase in pressure in the other of said chambers.

6. A power control apparatus of the vac directly ing a conduit having anormally closed main valve separating two chambers in WlllChthS pressurefluctuates in accordance with dif ferent sequences, respectively, aminimum flow ofair from one chamber to the other normally beingpermitted, a pneumatic for opening the main valve upon the occurrence oia predeterminec decrease in pressure in one of the chambers while thevalve is closed, and means for initiating closing of the valve upon theoccurrence of a predetermined pressure increase in the other of saidchambers occurring while the valve is open." i

7. A power control apparatus for a pneumatic system of the vacuum typehaving an open ended transmission tube and a suctiontube, said apparatuscomprising a. conduit providing chambers communicating respectively withthe transmission and suction tubes, a valve normally separating saidchambers, a minimum flow of air from one chamber to'the other normallytaking place, a-pneumatic actuable bythe pressure decrease in thetransmission tube occasioned by introduction of a carrier for openingthe valve, and a second pneumatic directly actuable by increase inpressure in the suction tube incident to delivery of a carrier from thetransmission tube to initiate closing of the valve.

8, A power control apparatus for a pneumatic system having atransmission tube and a suction tube comprising a casinghav ing chambersconnected respectively with the transmission and suction tubes, a valvenormally closing a passage between the chambers, a minimum flow of airnormally being permitted from the transmission to the suction tube,means operating upon the occurrence of'a dropin pressure in thetransmission tube incident to introduction of a carrier therein toopen'the valve and to maintain it open during the flight of the vcarrier, and a pneumatic actuable by increase in pressure in the suctiontube incident to delivery of thecarrier from the transmission tube torender the valve opening means inoperative.

' 9. Power control apparatus having a transmission tube'and anexhauster, said apparatus comprising a conduit providing chamberscommunicating respectively with the tube and exhauster, a valvecontrolling a passage connecting said chambers, a pneumatic actuatedbypressure change in-the first of said ch'ambersfor initiating movementof the valve in one directionfand a second pneumatic subject J topressure change inthe other of said chambers, to initiate movement ofthe'valve in the opposite dire'ction.

i I Power control apparatusfora pneumatic dlspatch system of the vacuum,1111111- mum flow type, comprising a casing-"having two chamberstherein, a main valve controlling a passage connecting said chambers,a'pneumatic for moving the valve in one direction upon drop in pressurein one of said chambers, a-normally closed auxiliary valvefor admittingair at atmospheric pressure to act on one sideof said pneumatic'to causethe valve to move in the opposite direction, and a second pneumatic,responsive to pressure increase in the other of said chambers,foriopening the auxiliary valve.

11. Power control apparatus for a pneumatic dispatch system of thevacuum, minimum flow type, having a transmission tube and a conduitconnecting said tube with an exhauster, said conduit comprising twochambers, a main valve controlling a passage connecting said chambers,means tending to close the valve, a pneumatic operable upon" drop inpressure in one of said chambers tofopen the valve, a normally closedauxiliary valve'for admitting air to a space at one side of saidpneumatic, and an auxiliary pneumatic sensitive to rise in pressure inthe other chamber to determine opening of the auxiliary valve. I 1

12. Power control apparatus comprising a casing, a main valvecontrolling communication between two chambers within the casing, aspring tending to close the main valve, a pneumatic connected to thevalve, said pneumatic opening the valve uponde crease in pressure inoneof saidfchambers, a normally closed auxiliary valve for admitting airat atmospheric pressure to a space atone side of said pneumatic, asecondpneumatic, subject to low pressure normally prevailing in the otherchamber, normally holding the auxiliary valve closed, and a springtending to open the auxiliary valve.

13. Power control apparatus for a pneumatic transmission tube comprisinga casing having a pair of cha'mbers therein, a main valve controlling apassage" connecting said chambers, means tending to keep the valveclosed, a minimum flow of air past theclosed valve being'permitted,'apiston connected to the valve, said piston being subject to the pressureprevailing in one of said chambers and becoming operative to open thevalve when said pressure drops to a predeter- *mineddegree, an auxiliaryvalve for admitting atmospheric pressure to act on one side of saidpiston, aspring tending to open said auxiliary valve, and apressureoperated element subject to low pressure normally preva'iling-inthe other of said chambers and normally holding the auxiliary valveclosed in opposition 'to 1ts spring.

14. Power. control apparatus for a pneumatic transmission tube comprismga casing yhaving'an elongate hollow bodyportion pro vided atone end withmeans for connecting it to the transmission tube and at its other endcommunicating with a source of low pressure, a piston valve guided formovement transversely of the casing and controlling the passage of airthrough the latter,

and means tending to hold the valve closed,

the valve having a head subject to pressure conditions obtaining in thetransmission tube, drop in pressure in said tube causing the valve toopen.

15. Power controlapparatus for a pneumatic transmission tube comprisingan elongate hollow casing conmiunicating at one end with thetransmission tube and at the other with a source of low pressure, pistonvalve sliding transversely of the casing, a spring tending to close thevalve whereby to obstruct the passage of air through the casing, ahollow arm projecting from the casing providing a chamber, the head ofthe valve constituting a movable wall for said chamber, a passageconnecting the chamber with the transmission tube end of the casing, anda normally closed passage connecting said chamber with the outer air.

16. Power control apparatus for a pneumatic transmission tube comprisingan elongate hollow casing connected at one end with the transmissiontube and at its other end with a suction tube, oppositely extendinghollow arms projecting from the casing, piston valve slidably guidedwithin said arms, means tending to more the valve to a position toobstruct the flow of air through the casing, means normally establishingthe transmission tube pressure in one of said arms to act upon theheadof the piston valve, a normally closed auxiliary valve for admittingatmospheric pressure to the interior of said arm, and means controlledby the pressure obtaining in the suction tube for determining opening oisaid a xiliary valve.

17. Power control apparatus comprising a casing having an elongateportion connect-- ed at its opposite ends to a transmission tube and asuction tube respectively, the casing also having a lateral chambercommunicating with the transmission tube, a piston having a head forminga movable wall for said chamber, the body ofthe piston constituting avalve, means tending to move the piston to a position to obstruct theflow of air longitudinally through the casing, a normally closedauxiliary valvefor admitting atmospheric pressure tothe lateral chamber,a spring tending to open the auxiliary valve, and a piston connected tothe auxiliary valve, the'head of said piston being exposed on one sideto the pressure obtaining in the suction tube.

18. Powerv controlapparatus for a pneumatic transmission tube comprisinga hollow casing communicatingat one end with the transmission tube andat its other end with a suctiontube, a piston valve sliding transverselyof the casing, means tending to close the valve to; cut off air flowthrough the casing, and an adjustable stop to limit closure of thevalve. 7

19. Power control apparatus for a pneumatic dispatch system of thevacuum having an open ended transmission tube, said apparatus comprisinga main valve, means tending to close the valve whereby to obstructpassage of air through the. tube, and adjustable stop preventingcomplete closure of the valve whereby to provide for a normal minimumflow of air through the tube, means for opening the valve, and anadjustable stop to limit the degree of opening of the valve.

20. Power control apparatus for a pneumatic dispatch system having atransmission tube, said apparatus comprising a casing connected to thetrans-mission tube, a piston valve slidable transversely of the casing,meanstending to close said valve whereby completely to cut oil flow ofair through the casing and tube, and a stop engageable with the valve toprevent it from closing completely. I V

21. Power control apparatus for a pneumatic dispatch system having atransmission tube, said apparatus comprising a casing having apassageforming an extension of the tube, the casing being provided with acylindrical guideway extending transversely of the passage, apistonvalve sliding in the guideway, and a fixed rodextending longitudinallyof the guideway eccentrically oi' the axis of the latter, the pistonvalve having sliding engagement with the rod whereby the latterpreventsrotation of the piston valverelatively to its guideway,

22. Power control apparatus for a pneumatic dispatch system having atransmission tube, said apparatus comprising a reciprocal piston valvefor; controlling air flow through the tube, said valve having acylindrical shell provided with ahead at one end and open at the other,a hollow axially disposed stem united to the head and projecting outthrough the open end of the shell, and a second head member secured tothe outer end of the stem. 7

23. Power control apparatus for a pneumatic dispatch system having atransmission tube, said apparatus comprising a conduit forming acontinuation of the tube, a main valve normally obstructing said conduitand dividing it into inlet and outlet chambers, a pneumatic for openingthe valve, said pneumaticbeingnormally subject to the pressure obtainingin the inlet chamber, an auxiliary valve for controlling the admissionof air at atmospheric pressure to act on said pneumatic, a secondpneumatic exposed upon one side to the pressure obtaining in the outletchamber, a spring acting in opposition to said last pneumatic andtending to open the auxiliary valve, and

means for retarding the closing of the auxiliary valve.

Power control apparatus for a pneumatic dispatch system, of the vacuumminimum flow type said apparatus comprising a main valve for controllingthe flow of air through a portion of the system, a pneumatic for movingthe valve, means for admitting air to operate said pneumatic, said meanscomprising a normally closed auxiliary valve, a spring tending to openthe auxiliary valve, a piston connected to the valve, and a closedcylinder in which the piston slides, said cylinder having an inletpassage of restricted capacity in its head.

25. Power control apparatus for a pneumatic dispatch system, saidapparatus comprising a main valve for controlling the flow of airthrough a portion of the system, a pneumatic for moving the valve, meansfor admitting atmospheric pressure to act on said pneumatic comprising anormally closed auxiliary valve, a piston connected to said valve, aclosed cylinder in which the piston slides, one head of the cylinderbeing provided with a small inlet passage and a relatively large outletpassage, and a check valve preventing entrance of air through theout-let passage.

26. Power control apparatus for a pneumatic dispatch system comprising acasing having an inlet chamber and an outlet chamber, a valvecontrolling flow of air from one of said chambers to the other, apressure operated element for moving the valve, one face of said elementforming a movable wall for a space connected by a passage with the inletchamber, said casing having another passage leading from said space tothe atmosphere, an auxiliary valve normally closing the latter passage,a secondary pressure controlled element forming a movable wall for aspace communicating with the outlet chamber of the casing, and meansconnecting said latter pressure controlled element with the auxiliaryvalve.

27. Power control apparatus for a pneumatic dispatch system comprisingacasing having an inlet chamber and an outlet chamber, a valvecontrolling communication between said chambers, a piston for moving thevalve, said piston constituting a movable wall for a space within thecasing, said space being connected by a passage with the inlet chamber,adjustable means for restricting the capacity of said passage, a secondI JAMES G. MACLAREN

